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The Leading Edge; April 2003; v. 22; no. 4; p. 344-350; DOI: 10.1190/1.1572088
© 2003 Society of Exploration Geophysicists
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The impact of water-velocity variations on deepwater seismic data

Scott MacKay and Jonathan Fried

WesternGeco, Denver, Colorado, U.S.

Charles Carvill

WesternGeco, Houston, Texas, U.S.

Corresponding author: smackay1@denver.westerngeco.slb.com

The first 20% of the full text of this article appears below.

During marine seismic acquisition, obtaining complete subsurface coverage may require combining data from different acquisition dates. The time gaps between the overlapping coverage may vary from hours separating subsequent boat passes, to months when large surveys are acquired in sections. Time-lapse data are an extreme example of overlapping data sets acquired at widely varying acquisition dates.

Unfortunately, between the different times of acquisition, changes in physical ocean properties, such as temperature or salinity, can cause variations in water velocity. The result is a dynamic change in recorded traveltimes that makes accurate combination of the data difficult. In shallow water, the distortions are small and do not affect data quality. However, in deepwater, the cumulative distortions can pose a serious impediment to accurate imaging.

The existence of water-velocity variations has been documented previously (Barley, 1999). Water temperature changes are the primary cause of velocity variations. Figure 1 shows an area just south of Nova Scotia (coastline in red). The outlined seismic survey area is approximately 3600 km2. The satellite images show surface temperature variations, with each color contour representing 1°C. In the approximately two-week period shown, surface temperatures varied as much as 10°.


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  		Figure 1. Surface temperature variations offshore Nova Scotia; each color contour is 1°C. The survey area is outlined. In the approximately two-week period shown, up to 10° of temperature variation can be seen.

 
The temperature structures evident in Figure 1 are caused by eddies in the Gulf Stream and are indicative of deepwater temperature variations. Significantly, each degree of change causes over 3 m/s of water-velocity variation. The effect of such changes on seismic data collected in deepwater can be significant. Figure 2 shows two midpoint gathers after moveout correction. The gathers are from the Nova Scotia survey outlined in Figure 1. The shallowest event is the water-bottom . . . [Full Text of this Article]




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